The nitrate oficina (saltpeter mine) of Marusia was located in the foothills of the Andes in the región de Tarapacá, about 41⁄2 miles northwest of the town of Huara.

The union, under the leadership of Domingo Soto (labor leader), was afraid of reprisals, especially another massacre like the one that had happened only a few years before in San Gregorio, so they decided to take several preventive measures.

Upon being apprised of the unrest, the government responded by sending forty soldiers under the command of Captain Gilberto Troncoso, known as the "Hyena of San Gregorio" for his violent behavior.

In an open assembly, the union leaders proposed negotiating their surrender, while some miners advocated calling upon the help of the workers from other mines.

A group of workers was able to mount a hasty defense, throwing dynamite sticks at the advancing troops, and they managed to kill 36 soldiers and injure 64.